My 2 cents and experience with this,
There is generally a period where we get big shad in the west coast, so either they come in to breed or they are returning from where they spawned.
Places I have encountered or heard of shad along the west coast is, Melkbos, ganzekraal, langebaan, Saldanha bay,Jacob's bay, Veldrift, Standfontein(past Doorn baai) so its quite expansive yet limited at the same time.
The small shad are dominant from August till say Beginning May. But they are always hammered at Veldrift as they come in to the river system for feeding and protection from predators.
A lot of the bigger shad I have seen come out is between Langebaan and Saldanha bay mostly on the boats.
I have heard of the fish going into the reserve in the lagoon during winter, but I honestly feel they just go to the deep waters and travel up and down the coast lines.
I feel with the red tide that occurred years back in the west coast, it left devastating effects on the fish population and recovery has been slow with all the netting going on around the coast.
I have to mainly direct this issue towards Veldrift social behavior, as I have personally seen an incredible difference in the fishing when the netters got nailed over a year back with the edibles at Soverby. So if daff had to invest in a proper marine poaching security it would attract a much healthier fishing system with fish coming in to spawn and have that spawned fish survive to be able to spawn more eggs later. Hopefully the marine protection zones will help with this.